Most prepared food products, after they are prepared and ready for mass distribution through retail outlets, must be packaged, distributed and sold in a manner to prevent deterioration over prolonged periods of time, referred to as "shelf life". Over the years the food industry has developed many packaging and distributing processes for accomplishing the objective of instilling food substances with a long shelf life. Cans, jars, frozen or refrigerated containers, and hermetically sealed plastic packages or containers are the more common processes and packaging for this purpose. These procedures have presented certain disadvantages generally associated with either the expense of the packaging or the cost of refrigerating, freezing and maintaining the pressuring condition of the food packages as they progress through the retail channels. In view of this situation, there has been a tremendous effort in the food industry to develop inexpensive, serving size containers for prepared foods which need not be frozen and/or refrigerated to accomplish an acceptable shelf life. One of the more successful approaches in the quest by the food industry to accomplish this objective has been the development of a plastic container having an appropriate gas barrier layer to prevent oxygen migration through the walls of the container and incorporating an upper lid formed from a laminate and also having an oxygen migration barrier which is heat bonded by an appropriate bonding material layer over the access opening of the plastic container. By merely placing the prepared food into the container and heat bonding the lid thereto, oxygen cannot enter the container through the container itself or through the upper lid. By removing oxygen from the container before the lid is sealed onto the container either by drawing a vacuum, purging the container with asn iner gas and/or both of these procedures, oxygen is excluded from association with the packaged food product to accomplish a long, acceptable shelf life. This new packaging concept is now becoming quite common and is used for many products. In accordance with one known procedure, the laminated lid is sealed onto the container by employing a heated ring forced against the lid so that the heat energy of the ring is conducted to the layer of heat bondable material between the lid and container flange whereby the heat bondable layer is heated and seals the lid onto the container in a path of "footprint" defined by the shape of the ring. Although this procedure does operate and is being widely employed in the food industry, there are distinct disadvantages. Heating of the ring for conducting heat energy from the ring to the heat bondable material cannot be accomplished rapidly. In addition, the ring does not cool rapidly. It is critical to keep the ring in place until the seal is set. Consequently, the heating cycle for heating and allowing the ring to cool is relatively long. Further, uniform heating around the surface of the ring and adjustment of the heating effect to accomplish optimum sealing is difficult, if not impossible, to accomplish.
In view of the substantial disadvantages associated with a heated ring to heat the bonding material by conduction for sealing the lid onto the container, a relatively new technique has developed wherein the ring is an inductor and a metal layer is incorporated into the laminated sheet material forming the lid, so that as a high frequency is applied to the inductor, the metal layer in the lid material itself is heated only in the area adjacent the inductor. By forcing the inductor against the lid, induction heating of the metal in the lid directly below the inductor causes sealing of the heat bondable material sandwiched between the lid and the container. Immediately upon de-energizing the inductor, heat is removed from the metal layer of lid and final sealing is accomplished. In this manner, accurate control may be maintained over the heat sealing function so that the lid can be economically and rapidly applied onto the container to accomplish sealing of the container in a fashion which can be repeated from one container to the next. Mohr's U.S. Pat. No. 4,707,213 illustrates induction heating of a metal layer for the purpose of sealing a lid onto the container which has proven successful, reliable and repeatable.